In the refining of hydrocarbon feedstocks, the feedstocks are distilled into fractions, and various fractions and fraction mixes are often in turn subjected to cracking operations to produce methane, olefins and other gases. Contaminants in the crude hydrocarbons subjected to the refining operations include acids or acid-forming materials such as CO.sub.2, H.sub.2 S, CH.sub.3 SH, HCl, and the like. The problems of corrosion of metal surfaces which come into contact with these acids or acid-forming materials are all too well known to the refining and processing arts.
These acid-forming materials must be removed from natural and cracked gas streams (which contain such hydrocarbons as methane, olefins such as ethylene, styrene, butadiene, cyclopentadienes, etc.). One method of removing the acid gases is separating them from the hydrocarbon gases by absorption in an alkanol amine regenerative aqueous absorbent system. Regenerative alkanol amine aqueous absorbent units, as more fully described below, include columns with trays or other packing which are used to contact the aqueous alkanol amine solution which the acid-forming gases, and also include heat exchange surfaces which are normally used to conserve energy and regenerate the absorbent. The acid gases create corrosion and fouling problems in these alkanolamine units. Especially where velocity is low, such as around trays and in heat exchangers or at other outlets, fouling of the passages by ferrous oxides and insoluble acid salts of ferrous metals, such as ferrous sulfides and carbonates, occurs. The reactions which give rise to ferrous metal particulates are reactions which also surface erode ferrous metal through corrosion. In addition, the amines groups of the alkanol amines react with the acidic gases to form heat-stable salts which also form fouling deposits on the equipment. The acid salts of amines, both soluble and insoluble, require treatment of the system with caustic in order to maintain the pH at a level which reduces the severity of acid-induced corrosion. The presence of fouling agents requires that the system be shut down periodically for internal cleaning. Shutdown of a unit means lost capacity and is in addition, itself, an expensive, time-consuming operation.
The addition of chemicals to the system to reduce fouling or corrosion is generally a more economical and desirable method of dealing with the fouling or corrosion problem than plant shutdowns, and consequently, many chemicals have been investigated for anti-foulant or corrosion-resistant activity.
A particular problem with acid gas-contaminated gas feeds which contain unsaturated hydrocarbons such as those mentioned above is the formation of polymer gums in the alkanol amine equipment. In U.S. Pat. No. 4,575,455, hydroxyl amines were disclosed for use as an anti-foulant to combat the buildup of polymeric material resulting from the thermal polymerization of unsaturated hydrocarbons processed in alkanol amine gas-scrubbing equipment. The hydroxyl amine compound functioned as an oxygen scavenger for trace amounts of oxygen in the system that were effective to accelerate the polymerization giving the foulant gums, and it was also mentioned that oxygen would promote the sloughing of iron sulfide off walls of the unit, increasing the amount of iron sulfide available to foul reboilers.
In U.S. Pat. No. 4,490,275, the occurrence of amine-salt deposits in equipment was addressed by the use of dimethylaminoethanol and dimethylisopropanolamine as an alkanol amine. In U.S. Pat. No. 4,647,366, a composition of a reaction product of an alkynediol and a polyalkylene polyamine was stated useful to inhibit corrosion from propionic acids in the essential absence of water but to be ineffective to protect ferrous metal surfaces from corrosion from inorganic gases, such as hydrochloric acid, or even from acetic acid.